The function of a positioner when used as a safety instrument (for example, when used as a safety instrument in a fluid valve in a process control system) is to facilitate the actuation of a final control device from a normal operating state to a safe state (e.g., a tripped state). In some example valves, the actuation of the final control device occurs in response to a setpoint signal going from high to low (e.g., deenergize to trip (DETT)). In other example valves, the actuation of the final control device occurs in response to the setpoint signal going from low to high (e.g., energize to trip (ETT)).
Safety systems in which valves fail or degrade (e.g., a component of the field device fails or degrades) may result in the inability of the device to achieve the safe state. Valve failure/degradation during operation can also create undesirable operating conditions of the safety instrumented system. The possibility of a failed valve creating undesirable operating conditions can be mitigated by implementing redundant safety instruments in the valve.